The present invention relates generally to the use of a differential interferometer to measure small differences in optical absorptivity between two weakly absorbing samples located therein, and more particularly to the use of the chanqe in refractive index of a weakly absorbing sample due to the heat generated when an intense laser beam traverses it relative to a second irradiated sample of slightly different optical absorptivity, each of which is positioned in one arm of an interferometer cavity, which change produces a fringe pattern shift which can be related to the difference in optical absorptivity of the samples.
In many instances the detection of trace amounts of dissolved material is complicated by large background absorptions of the solvent. Under such conditions the part of the absorption signal due to the solute can be much smaller than the precision of the measurement of the combined solvent and solute absorption in which case no useful concentration information can be obtained using conventional measurement technology. The apparatus and method of the instant invention allows the solvent absorptivity to be nulled out of the measurements, the resulting absorption related signal being related solely to the solute absorptivity. A cell containing a weakly absorbing solution is placed in each arm of an interferometer. Heat is generated when a laser beam traverses the cells since a certain fraction of its energy is absorbed by the sample causing an increase in temperature. This results in a change in the index of refraction of the solution which is temperature dependent with a consequent change in the optical pathlength of the laser beam through the cell. Such changes in pathlength can be measured with an interferometer. For low-loss media (losses of 10.sub.-4 cm.sup.-1 or less), this effect is very small, and sensitive interferometers in well-controlled environments must be employed. For solutions having the same absorptivity, and for heating beams of equal intensity, the change in optical pathlength produced by laser heating is the same in each cell, and there is no shift in the fringe pattern formed by the interferometer. However, any difference between the absorptivities of the two solutions results in unequal heating and a fringe pattern shift. The degree of fringe movement is a measure of the difference in absorptivity between the two cells. A critical feature of the instant invention, which allows clear resolution of differences in sample absorptivity of approximately 10.sup.-5 cm.sup.-1, is that two fringe patterns are formed using laser beams from the interferometer. A signal fringe pattern is formed by a pair of low-power helium-neon laser beams each of which traverses a sample cell colinearly with a heating laser beam. A reference fringe pattern is formed from another pair of low-power helium-neon laser beams passing through the sample cells narrowly displaced from the heating beams. The purpose of this latter fringe pattern is to stabilize the signal fringe pattern with respect to normal thermal drift. This stabilization is achieved using a movable compensator plate inside the interferometer controlled via a feedback loop which has as its input the fringe pattern intensity at a fixed point which is a measure of the fringe position. It is this stabilization which allows the achievement of the minimum detection limits to be reported hereinbelow, and which represents a significant improvement over that which can be achieved by other methods where the solvent absorptivity is in the range of about 10.sup.-3 cm.sup.-1 or higher using the instant invention. Signals due to solvent absorptivity have been nulled to approximately one part in four hundred which thereby allows the clear resolution of absorptivity differences of about 10.sup.-5 cm.sup.-1 using about 50 mW of laser power. For comparison in the nondifferential configuration of the instant interferometer; that is, a sample cell in only one arm, and with a background absorption of 10.sup.-3 cm.sup.-1, the minimum detectable change in solute absorption is approximately 10.sup.-4 cm.sup.-1 in the best cases. The decrease in minimum detection limit which occurs with differential detection is due principally to the increased precision of the data relative to that obtained using nondifferential techniques.
Although interferometers (refractometers) have been used for over one hundred years to compare the refractive indices of gases, the use of an interferometer to detect weak absorptions using the thermooptic effect was only recently demonstrated.
1. In "Measurements of the Absorption of Light in Low-Loss Liquids," by J. Stone, J. Opt. Soc. Amer. 62, 327 (1972), the author describes a dual-beam interferometer where an intense laser light beam simultaneously heats the single solution under investigation and monitors its change in optical pathlength due to heating and thermal drift, while another laser beam of weak intensity monitors any changes in optical pathlength due to thermal drift alone. Both beams in Stone's apparatus are separated by only a few millimeters and traverse the same solution so that the change in optical pathlengths from drift recorded by the beams are equal. In this way the fringe pattern was stabilized with respect to drift and small shifts in this pattern due to heating could readily be measured. The apparatus could not, however, be used for differential measurements since the fringe pattern is not stabilized if both beams are used to record the thermooptic signal from different solutions. This is the critical feature of the instant invention, not taught by Stone. Our invention teaches the use of a drift stabilizing reference laser beam in each of two sample cells for which the difference in absorptivities is being determined, the stabilization of the fringe pattern resulting therefrom leading to significantly enhanced measurement sensitivity. The instant invention, then, teaches the measurement of changes in refractive index of the two sample solutions due to heating using weak intensity probe laser beams colinear with heating laser beams. Thermal drift in the probe fringe pattern is reduced by the use of a reference set of fringes formed from low-intensity reference laser beams which pass through the sample solutions slightly displaced from the probe and heating beams. A driven compensator plate inside our interferometer is connected to a feedback loop which responds to error signals resulting from thermal drift, which signal is used to keep the reference fringe pattern stable. The drift in the probe fringes was found to be identical to that of the reference fringes due to the proximity of the two sets of beams so that both sets of fringes could be stabilized in this manner, without which the reported sensitivity could not have been achieved.
2. In "Optical Nulling for Trace Analysis Based on The Thermooptic Effect and an Interferometer," by David A. Cremers and Richard A. Keller, Advance Program of the Conference on Lasers and Electrooptics, Washington, D.C., June 10-12, 1981, distributed sometime in April, 1981, and in "Quantitative Measurement of Solute Absorption at Levels Below Solvent Absorption by Optical Nulling Techniques," by D. A. Cremers and R. A. Keller, Los Alamos Conference on Optics '81, Santa Fe, NM, Apr. 6-9, 1981, the authors, who are also inventors of the instant invention, briefly describe the invention in abstract form. However, although both references mention the differential nulling procedure, neither even hints at the fringe stabilization critical in attaining the minimum detection limit quoted hereinabove.
An object of the instant invention is to measure small differences in optical absorptivity between two weakly absorbing samples.
Another object of our invention is to determine the concentration of a solute in solution where the solute absorptivity is much smaller than that of the solution.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus of this invention may comprise an interferometer cavity into which is placed two cells containing the samples under investigation, narrowly-spaced probe and reference laser beams which are each split into two widely-spaced, parallel beams after entry into the cavity, one probe beam and one reference beam becoming narrowly-spaced and parallel before entering one of the sample cells, and the other probe and reference beams becoming similarly narrowly-spaced and parallel before entering the second of the cells, the two probe and two reference laser beams being recombined to form a probe and reference fringe pattern, respectively, two amplitude modulated heating laser beams, means for equalizing their intensity and rendering them colinear with the split probe laser beams, one with each, and means for separating them from the probe beams after they exit the sample cells, means for equalizing the intensity and adjusting the parallelism of the two beams in each of the probe and reference beam pairs, means for stabilization of the reference and probe fringe patterns, and means for detecting and recording the probe fringe pattern intensity. Preferrably, the interferometer cavity is of the Jamin design in order to reduce sensitivity of the apparatus to vibrations. It is also preferred that the probe and reference laser beams incident upon the interferometer cavity derive from a single laser source the output of which is divided into two approximately equal intensity parts. Similarly, it is preferred that the heating laser beams derive from a single laser source, the output of which is divided into two approximately equal intensity parts. It is further preferred that each pair of narrowly-spaced probe and reference laser beams be located approximately 5 mm apart. It is finally preferred that the means for stabilizing the reference fringe pattern include a movable compensator plate which is part of a closed feedback loop which further includes means for dithering and driving the plate to provide modulation and adjustment of the fringe pattern necessary for phase-sensitive detection. The compensator plate intersects the probe and reference beams in one arm of the interferometer so that the shift in probe fringes due to a thermooptic-induced response of the interferometer is also modulated at the same frequency as the reference fringes and can be monitored by phase-sensitive detection.
In a further aspect of the present invention, in accordance with its objects and purposes, the method hereof may also comprise splitting incident parallel probe and reference laser beams into a pair of probe and reference laser beams, each widely spaced split probe beam of approximately equal intensity traveling in a parallel direction, and similarly for the split reference beams, one probe and one reference beam emerging narrowly-spaced, and similarly for the second probe and reference beam, inserting a light-transmitting sample holding cell in the path of each pair of laser beams, recombining the two probe beams and the two reference beams to form probe and reference fringe patterns, respectively, introducing the samples under investigation into the cells, superimposing an amplitude modulated heating laser beam to be colinear with each probe laser beam before it enters the sample cell for a specified period of time, each of these heating beams having substantially the same intensity, separating the laser heating beam from each probe beam after each pair has passed through one of the sample cells, stabilizing the reference fringe pattern with respect to drift due to air currents, variations in room temperature, and thermal gradients in the sample cells thereby simultaneously stabilizing the probe fringe pattern, monitoring the probe fringe pattern for movement during the time that the heating laser beams pass through the sample cells, differences in absorptivity of the contents of the cells causing different degrees of heating in each cell as a result of the thermooptic affect thereby changing the refractive index of that cell's contents and consequently its optical pathlength for the probe beam traversing it which in return varies the fringe pattern position, establishing the relationship between the difference in absorptivity of the two samples and the position of the probe fringe pattern with samples of known absorptivity, and relating the position measured for the samples under investigation to a value of this difference in absorptivity. It is preferred that two pairs of narrowly-spaced probe and reference laser beams be separated by about 5 mm in order that the stabilization of the reference fringe pattern will simultaneously stabilize the probe fringe pattern. It is also preferred that the specified period of heating be approximately 10s followed by an approximately 1 min. period of cooling before another heating period and measurement of the consequent probe fringe pattern intensity change is commenced. Preferrably, the probe and reference laser beam splitting and recombining steps are performed using a Jamin-type interferometer in order to make the sensitivity of the absorptivity difference measurements to vibrations minimal. It is finally preferred that the stabilizing of the reference fringe pattern position be accomplished using a fringe position detector, feedback electronics, and a driven compensator plate which intercepts one pair of probe and reference beams.
In summary, then, an apparatus and method for the measurement of small differences in optical absorptivity of weakly absorbing samples using differential interferometry and the thermooptic effect has been developed. The critical feature of our invention is the stabilization against drift of the optical pathlength of the probe laser beams which interrogate the samples under investigation, which differential change in pathlength is related to the absorptivity difference between the samples. It is thereby possible to null out the bulk of the sample absorptivity and measure only this difference with substantial precision. Solute absorptivities of 10.sup.-5 cm.sup.-1 have been measured in solutions with total absorptivity in excess of 10.sup.-3 cm.sup.-1 which is not possible with nondifferential methods. Further, the smallest absorption measured with our invention was about 5.times.10.sup.-6 cm.sup.-1, which is comparable to or below measurements made using other thermooptic techniques. The principal advantages of the instant invention include low detection limits, only one critical alignment--that of the super-position of the probe and heating laser beams, simple analysis of the data, insensitivity to small variations in laser beam profile since the technique does not depend on the formation of a thermal lens, and easily obtained differential thermooptic spectra of solutions. This last use of our invention involves the use of separate probe and heating beams to record the thermooptic signal which permits scanning of the heating wavelength without the problem of phase shifts introduced by dispersion in optical elements through which the beams pass. Since the wavelength of the probe beam is fixed, a phase shift occurs only due to heating of the solutions.